Work around bug in QTextLine::xToCursor

With RtL text, the method can be non-monotonic wrt x value (which is a
Qt bug). To work around this, we check whether positions adjacent to
those returned by xToCursor look better. Depending on whether the new
x position is too small or too large, we look backward or forward for
a better solution.

The performance is probably not great, but this is only needed for
user interactions, so the performance penalty should not be a problem.

Fixes #10569.

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp b/src/frontends/qt4/GuiFontMetrics.cpp
index 33df3cf1267a68c5f7a3e51084e150d65b6abc9c..585933a328a11d93903ab5be9089849b191f8af4 100644 (file)
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -272,9 +272,34 @@ int GuiFontMetrics::x2pos(docstring const & s, int & x, bool const rtl,
                           double const wordspacing) const
 {
        shared_ptr<QTextLayout const> tl = getTextLayout(s, rtl, wordspacing);
-       int const qpos = tl->lineForTextPosition(0).xToCursor(x);
+       QTextLine const & tline = tl->lineForTextPosition(0);
+       int qpos = tline.xToCursor(x);
+       int newx = static_cast<int>(tline.cursorToX(qpos));
+       // The value of qpos may be wrong in rtl text (see ticket #10569).
+       // To work around this, let's have a look at adjacent positions to
+       // see whether we find closer matches.
+       if (rtl && newx < x) {
+               while (qpos > 0) {
+                       int const xm = static_cast<int>(tline.cursorToX(qpos - 1));
+                       if (abs(xm - x) < abs(newx - x)) {
+                               --qpos;
+                               newx = xm;
+                       } else
+                               break;
+               }
+       } else if (rtl && newx > x) {
+               while (qpos < tline.textLength()) {
+                       int const xp = static_cast<int>(tline.cursorToX(qpos + 1));
+                       if (abs(xp - x) < abs(newx - x)) {
+                               ++qpos;
+                               newx = xp;
+                       } else
+                               break;
+               }
+       }
        // correct x value to the actual cursor position.
-       x = static_cast<int>(tl->lineForTextPosition(0).cursorToX(qpos));
+       x = newx;
+
        /* Since QString is UTF-16 and docstring is UCS-4, the offsets may
         * not be the same when there are high-plan unicode characters
         * (bug #10443).